This invention relates to the placement of fluids in subterranean formations of oil and gas wells, and particularly to the placement of fluids in connection with hydraulic fracturing.
In subterranean formations of oil and gas wells, stress barriers can be insufficient to contain hydraulic fractures made within the producing zone. This can lead to inefficient fracturing, with much of the treatment potentially stimulating non-productive zones. Vertical fracture growth out of the hydrocarbon bearing portions of the formation, either up or down, may result from hydraulic fracturing in such formations having little or no stress contrast between the formation layers. A particular problem is the unwanted fracturing or stimulation of water or undesirable gas producing zones.
Containment of these undesirable fractures has been accomplished by placing an artificial barrier along the boundaries of the fracture to prevent further fracture growth out of the producing zone. Containment of fracture growth has been attempted by placing proppants and fluids with different densities in the fracture. These techniques are unreliable due to the difficulty of providing proper barrier placement.
SPE 25917 suggests control of fracture height growth through the selective placement of artificial barriers above and below the pay zone. These barriers are created prior to the actual treatment by pumping low viscosity carrying fluid with a mix of different size and density proppants that settle to the bottom and/or float to the top of the fracture channel or both. Typically a viscous pad is pumped to create a fracture channel, followed with a 5-10 mPa-s fluid slurry carrying a mix of heavier proppant that settles to the bottom of the fracture channel and a light proppant that rises to the top of the fracture channel. The proppant bridges at the top and/or bottom of the fracture can block vertical fracture growth. However, the accurate placement of two kinds of proppant through control of density and viscosity of one carrier fluid can be a challenging task.
Selective treatment of fracture zones is known. For example, U.S. Pat. No. 5,425,421 injects a settable gel composition, such as a polyacrylamide polymer cross-linked with inorganic transition metals, into the portion of the fracture extending within a water-producing zone. Placement of two or more different fluids into a forming fracture had been reported before, although for purposes other than selectively treating fracture zones. For example, U.S. Pat. No. 5,411,091 describes a method for enhanced hydraulic fracturing, which involves injecting a proppant-laden fracturing fluid, then a low-viscosity spacer fluid, and then a proppant-laden fracturing fluid at a sufficient rate and pressure to hold the created fracture open. This allows proppant to be more evenly distributed throughout as it falls through the spacer fluid, thereby claiming to avoid proppant convection in the fracture while obtaining substantially improved propping of the fracture.
The use of particles in fluids of different densities for proper placement and prevention of undesirable fracture growth into the bare rock zones is disclosed in U.S. Pat. No. 7,207,396. After pumping a proppant-free pad, lightweight proppant-laden slurry is introduced into the formation. Either the fluid density of the pad fluid is greater than the fluid density of the proppant-laden slurry, or the viscosity of the pad fluid is greater than the viscosity of the proppant-laden slurry.
U.S. Pat. No. 7,213,651 describes injecting a first fracturing fluid into a formation, followed by a second fracturing fluid, to create extended conductive channels through a formation. The fracturing fluids can be different in density, viscosity, pH and the other related characteristics to allow for variations in the conductive channels formed. Proppants can also be included in one or both of the injected fluids. The method attempts to enhance fracture conductivity while minimizing proppant flowback typically associated with hydraulic fracturing techniques.
It is thus seen in the prior art that combinations of two or more fluids are introduced into a subterranean formation for different purposes that may include altering formation permeability, proppant placement control, flowback prevention, etc. However, in practice such methods have difficulty to achieve prompt and accurate placement of fluids with special functions and/or laden with special materials into a designated segment of the fracture. In particular, the mobility of specialized fluids inside the fracture may be restricted by high shear stresses developed at the interface of the specialized fluid with other treatment fluids when the viscosities of the contacting fluids are both relatively high.